The general principle of operation of an optical scanning device will first be described with reference to prior art FIG. 10. A beam L from a laser 1 passes through a cylindrical lens 2 which condenses the laser beam into a line that is incident onto planar reflecting surfaces formed on a rotary polygon mirror 3. The rotary polygon is generally a regular polygon shape and rotates, for example, in the direction indicated by the arrow R so as to repeatedly scan, by reflection, a light beam that passes through an imaging lens system 4a, 4b so as to illuminate an object 5 that is to be scanned.
In such an optical scanning device, as shown in FIG. 11(a), air currents shown by the arrow A, are generated due to the rotation of the polygon mirror 3 in the rotation direction R. There tends, however, to be a low pressure region just after the intersection of the planar reflecting surfaces 3a. This is because air that is flowing relative to one planar reflecting surface 3a due to the rotation tends to separate from the next adjacent planar reflecting surface 3a, causing a low pressure region immediately following the intersections of the planar reflecting surfaces 3a at the polygon corners 3b. This negative pressure forms eddy currents A2 within the region W of each planar reflecting surface 3a. Dust carried by the air currents A1 and A2 tends to be deposited onto, and to adhere to, the reflecting plane 3a, which in time causes a cloudy region D to form on the mirror surfaces, as shown in FIG. 11(b). The adhering dust mainly concentrates near the center in the axial direction, as shown in FIG. 11(b), which is an active region in terms of the reflecting surfaces 3a acting to scan the incident laser beam by reflection. Therefore, when dust adheres to the region D, the reflectivity of the mirror surface 3a is markedly reduced in the region D, which results in a lowering of the quality of output of the optical scanning device.
Accordingly, a technique wherein the concentrated adherence of dust is prevented by providing protrusions which lead the air currents outside the reflecting surfaces around a rotary polygon mirror is disclosed in Japanese Laid-Open Patent Application H9-197328. Moreover, a technique wherein the above dust-adhering eddy region is sent to a region not used for scanning a light beam, and the light beam is incident only onto regions which are free from dust adherence is disclosed in Japanese Laid-Open Patent Application H7-199106.
In the techniques described in the above publications, the arrangement of protrusions complicates the construction, and the existence of protrusions causes disorder in the air currents around the rotary polygon mirror. In the case of the rotary polygon mirror disclosed in Japanese Laid-Open Patent Application H9-197328, the disorder in the air currents is even a source of noise. Since the adhered dust narrows the effective reflection region of the mirror surfaces, a larger-scale rotary polygon mirror may be used to counteract this problem. However, this increases the size and weight of the optical scanning device, which is undesirable.